The present invention relates to a negative electrode for a nonaqueous electrolyte secondary battery having a positive electrode and a negative electrode which is capable of occluding and discharging lithium electrochemically, a separator which separates said electrodes from each other and a nonaqueous electrolyte, and to a nonaqueous electrolyte secondary battery. Specifically, the present invention relates to an improvement in the storage property and discharge capacity of the negative electrode and the nonaqueous electrolyte secondary battery.
A lithium secondary battery represented by a lithium ion battery containing a positive electrode material which is a lithium containing transition metal oxide composite, for example, lithium-cobalt oxide composite (LiCoO2), lithium-nickel oxide composite (LiNiO2), lithium-manganese oxide composite, and the like, and a negative electrode material containing lithium metal, a lithium alloy or a carbon material which is capable of occluding and discharging lithium ion has lately been attracting attention for a battery for portable electronic communication equipment, for example, compact video cameras, cell phones, notebook computers and the like. A lithium secondary battery which has a carbon material as a negative electrode material has actually been used.
When a carbon material is used as the negative electrode material, a maximum capacity of a battery is theoretically 372 mAh/g. Various materials have been tried to increase the capacity. Aluminum and aluminum alloys have lately attracted attention as an alternative to a carbon material because they have a theoretical maximum capacity of 1200 mAh/g.
However, particle size of aluminum and aluminum alloys is reduced by repeated occluding and discharging of lithium ion during charge and discharge cycles and charge-discharge cycle is shortened. Japanese patent publication (Laid-open) Sho 62-100942 discloses that a composition comprising a uniform mixture of alkali metal or alkali metal alloy and a conductive polymer is used for a negative electrode to improve discharge capacity and life cycle. However, this attempt cannot provide sufficient improvement.
Objects of the present invention are to decrease the reduction of particle size which accompanies charge and discharge of aluminum or aluminum alloy, to increase discharge capacity and to obtain a nonaqueous electrolyte secondary battery having a cycle property.
The present invention provides a negative electrode consisting essentially of aluminum powder or aluminum alloy powder coated with carbon and a conductive polymer. A binder is used to adhere the coated particles together. Since aluminum or aluminum alloy has a theoretical maximum capacity as high as 1200 mAh/g, a battery having aluminum or aluminum alloy as a negative electrode material can have a high capacity.
Carbon and the conductive polymer coating formed on the surface of the aluminum powder or aluminum alloy powder prevent particle size reduction of the powder because the carbon and/or conductive polymer is adhered with a binder and maintains the powder particles strongly together. Therefore, even when charge-discharge cycles are repeated, an excellent cycle property of the battery can be maintained.
A conductive network among particles is created by the carbon and conductive polymer and conductivity is maintained to increase availability of the active material and to obtain a high capacity of the battery. Discharge capacity of the battery is not reduced even when the carbon and conductive polymer are coated on the surface of an aluminum powder or an aluminum alloy powder because the carbon and conductive polymer are capable of occluding or discharging lithium ion.
Preferred as the carbon which forms a coating on the surface of the aluminum powder or aluminum alloy powder is one having a d-value of the lattice plane (002) of not more than 3.50 xc3x85 and a size of crystal lattice in a direction of c axis (Lc) of greater than or equal to 13 xc3x85. Preferred as the conductive polymer which forms a coating on the surface of the aluminum powder or aluminum alloy powder are polythiophene, polyaniline and polypyrrole. These carbons and conductive polymers can occlude or discharge lithium ion well.
Preferably, the amount of the aluminum powder or the aluminum alloy powder is at least 25 weight % and not more than 80 weight % of the total weight of the negative electrode, the amount of the carbon is at least 5 weight % and not more than 50 weight % of the total weight of the negative electrode and the amount of the conductive polymer is at least 5 weight % and not more than 50 weight % of the total weight of the negative electrode. If an excessive amount of the carbon and the conductive polymer is used, discharge capacity is reduced and if the amount of the carbon and the conductive polymer is not sufficient, particles of the aluminum and aluminum alloy powder are not strongly adhered.
If the median diameter of the aluminum powder or the aluminum alloy powder is larger than 30 xcexcm, it is difficult to mix the powder with other components and the electrode reaction is not uniform. On the other hand, if the median diameter of the aluminum powder or the aluminum alloy powder is smaller than 5 xcexcm, the disadvantages of a reduction of particle size easily result. Therefore, the preferable median diameter of the aluminum powder or the aluminum alloy powder is 5-30 xcexcm.
The amount of the binder is an amount sufficient to adhere the coated particles together and is generally an amount of up to about 1% by weight of the negative electrode material.
As an active positive electrode material, there can be used a lithium-containing transition metal composite oxide composite, for example, lithium-cobalt oxide (LiCoO2), lithium-nickel oxide (LiNiO2), lithium-manganese oxide (LiMn2O4), lithium-iron oxide (LiFeO2), and the like.
As a solvent of the nonaqueous electrolyte, an aprotic solvent, for example, ethylene carbonate (EC), propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like can be used alone or in the form of a mixture of two or more of these substances can be used. A preferred solvent is a mixture of EC and at least one solvent selected from DEC, DMC and EMC in a ratio of EC to the other solvent(s) of 5:95xcx9c60:40 by volume.
As a solute dissolved in the solvent there can be mentioned LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(SO2C2F6)2 LiN(CF3SO2)2 LiCF3(CF2)3SO3 and the like.
There is not any limitation with respect to the size and shape of a battery of the present invention. It can be thin or regular. It also can be in the shape of a prism, a cylinder or a coin.
The present invention is described below in detail in conjunction with certain examples. However, it is of course understood that the present invention is not limited to the following examples. The present invention can be modified within the scope and spirit of the appended claims.